The present embodiments relate to wireless mesh communication system and, more particularly, to enhanced broadcast transmission in Field and Personal Area Networks.
A wireless mesh network is a type of wireless communication system where at least one wireless transceiver must not only receive and process its own data, but it must also serve as a relay for other wireless transceivers in the network. This may be accomplished by a wireless routing protocol where a data frame is propagated within the network by hopping from transceiver to transceiver to transmit the data frame from a source node to a destination node. A wireless node may be a wireless access point such as a wireless router, a mobile phone, or a computer capable of accessing a wireless Field Area Network (FAN). In other applications, the wireless node may be an external security monitor, a room monitor, a fire or smoke detector, a weather station, or any number of other FAN applications for home or business environments.
A practical mesh network must maintain continuous network paths for all wireless nodes. This requires reliable network formation, reconfiguration around broken or interrupted network paths, and prioritized routing to ensure that data frames travel from source to destination along short yet reliable network paths.
FIG. 1 shows an exemplary wireless Field Area Network (FAN) of the prior art as disclosed in version 0v79 of the 2013 Wi-SUN Alliance Field Area Network Working Group, which is incorporated by reference herein in its entirety. The FAN includes an upper level Wi-Fi control circuit 160 which is directly connected to the internet and serves network nodes within the FAN via Wireless Area Network (WAN) Backhaul circuit 150. The FAN also includes Personal Area Network (PAN) circuits A through C. Each of PAN communicates with WAN Backhaul circuit 150 through respective Border Router Nodes (BR) 100, 120, and 130.
PAN A is an exemplary network that may be similar to PANs B and C. PAN A communicates with WAN Backhaul circuit 150 through Border Router Node (BR) 100. BR 100 communicates directly with Relay node (RN) 102 and with Leaf Node (LN) 114. Thus, BR 100 is a parent node of RN 102 and LN 114. RN 102 is a parent of RN 104 and communicates indirectly with LN 106 via RN 104. RN 102 also communicates directly with RN 108 and indirectly with RN 110 via RN 108. RN 108 also communicates directly with LN 112. RN 108 is a parent of both RN 110 and LN 112.
Once a network node enters a PAN, it may communicate with other nodes of the PAN by uplink transmission to a parent, by downlink transmission to a child, or by peer-to-peer transmission. The mechanisms for each type of transfer depend on whether the network supports transmission of periodic beacons. If the network produces periodic beacons, these may be used by network nodes for synchronization. Alternatively, a network node may not require synchronization and may transmit asynchronously. For either synchronous or asynchronous transmission, however, the beacon is still required for network discovery so that a node may initially join the PAN. Network communication within the PAN is accomplished by Medium Access Control (MAC) frames. These include beacon frames, data frames, acknowledgement frames, and MAC command frames.
PAN nodes use carrier sense multiple access with collision avoidance (CSMA-CA) for either synchronous or asynchronous transmission. Synchronous transmissions are aligned to a PAN beacon with a corresponding back off period. Asynchronous transmissions within the PAN are transmitted on an unslotted CSMA-CA channel. For asynchronous transmission, a node waits for a random back off period while listening to the channel. If the channel is busy, the node waits for another random back off period before trying to access the channel again. When the channel is idle, the node transmits the desired frame. A corresponding return acknowledgement frame without CSMA-CA confirms reception.
Channel hopping has been widely adopted for communicating between network nodes in many wireless and wireline communication systems. Channel hopping essentially involves transmitting signals on different carrier frequencies among many available sub-carriers at different instances of time. A pseudorandom sequence known to both the transmitter and receiver is usually used so that the intended receiver can listen on the correct channel. This improves communication robustness to external noise and helps counter jamming and eavesdropping. Multiple technologies such as Bluetooth and Digital Enhanced Cordless Telecommunications (DECT) incorporate channel hopping mechanisms. Channel hopping may be achieved through many different methods. Among the most common methods are synchronous channel hopping or Time Slotted Channel Hopping (TSCH) and asynchronous unslotted channel hopping as defined in IEEE 802.15.4e, which is incorporated herein by reference in its entirety. Many standards also exist that use such channel hopping MAC to define MAC protocols for different applications such as the Wi-SUN Alliance FAN.
Referring to FIG. 2, there is a diagram of unslotted channel hopping for network nodes A, B, and C. In unslotted channel hopping MAC, each node picks a hopping sequence and hops their receivers to different channels according sequence. Each node spends a specified time or dwell interval on each channel before hopping to the next channel. Many methods exist to track the unslotted channel hopping sequence of neighboring nodes within the PAN such as the FH-Discover method proposed by the Wi-SUN Alliance.
There are several problems, however, that may occur with various channel hopping communication systems. Unicast transmissions to a specific receiver are receiver directed in the sense that a node transmits a frame in the receiver's channel using CSMA-CA. Referring to FIG. 3, for example, Node A has a normal hopping sequence 1-2-3-4-5. Node B has a normal hopping sequence 1-5-2-4-3. Node A receives a data request from Node B on channel 2. Node A responsively transmits Data to Node B on channel 5. Node B receives the Data and transmits an acknowledgement (ACK) to Node A on channel 5. But the Data and ACK exceed the dwell period of channel 5 and cause Node B to temporarily abandon its normal hopping sequence on channel 2. If another node of the PAN transmits a data frame to Node B on channel 2 according to its normal hopping sequence, the frame will be lost, since Node B is still communicating with Node A on channel 5.
Another problem may arise during broadcast transmission within the PAN. Broadcast transmission occurs during broadcast listening slots as shown in FIG. 4. Each node advertises its broadcast listening slot which is transmitter directed and all other nodes who are interested in the broadcast transmissions can then listen to the advertised node's broadcast channel. When a device listens to another nodes broadcast slot, it has to tune its receiver to the advertised broadcast channel. This requires the node to deviate from its advertised unicast channel. The node then resumes its unicast channel hopping after completing the broadcast channel listening as though no deviation had occurred. For example, when node A listens to a broadcast by node B, other nodes in the PAN will not know that node A has suspended its unicast schedule. If another PAN node transmits frame to node A based on its advertised unicast schedule the frame will be lost.
Another problem occurs when there is no limit to a number of broadcast channels to which a node may listen. In this case, a node may spend much of its time listening on a broadcast channel. Therefore, it does not follow its unicast channel hopping schedule and may miss unicast transmissions from other nodes within the PAN. A similar problem occurs when a node listens to only a few other node broadcasts to maximize time on its unicast schedule. In this case it may lose required broadcast data from other nodes. For example, nodes may lose routing information such as Routing Protocol for Low-Power and Lossy Network (RPL) frames. Thus, the node may not be able to choose a better parent when available. One solution is to have a single global broadcast schedule for all nodes to follow. But this limits broadcast transmissions to a single schedule and requires precise time synchronization of all nodes.
Although network proposals of the prior art provide steady improvements in wireless network communications, the present inventors have recognized that still further improvements in mesh network protocol are possible. Accordingly, preferred embodiments described below are directed toward this and other improvements over the prior art.